Image sensors and in particular video sensors with image intensification tubes are employed in low light areas to monitor scenes with minimal or no illumination. For example, such image sensors may be used for night-time surveillance cameras in high crime areas, for hand-held video camcorders which operate without aid of attached illumination devices or in conjunction with aerial drones which view a scene to confirm the location thereof. These sensors produce an image of a scene with enhanced brightness without introducing spurious brightness variations or noise therein.
Unfortunately, these aforementioned sensors are ineffective where a bright contrasting light enters the scene or where rapidly changing scenery is viewed. A bright light in a previously dimly-lit scene causes video images to smear and become distorted. Rapidly changing scenes, which occur with drone-mounted sensors, are incapable of quickly adjusting to the varying light levels viewed by the sensor. In other words, contrasting artificial light sources tend to integrate and obscure images without providing any additional features in the image. This problem is further exacerbated in drones which employ temporary illumination to confirm their in-flight position.
One attempt to overcome this problem is to perform an integration operation on the current supplied to the image intensification tube by using only a part of each image to set operational characteristics. However, this approach employs only the image intensification tube in the compensation loop and does not consider the operational parameters of the tube or other components within the sensor. Moreover, each component within a sensor must be individually calibrated for its particular operating characteristics to obtain a useable output. Of course, this adds cost and production time to the manufacture of the sensor. Additionally, the output of known sensors may be adversely affected by degradation of a single component therein.
Based upon the foregoing it is apparent that there is a need in the art for a sensor which automatically maintains the desired operational parameters in video imagery while the sensor is viewing dimly lit and/or rapidly changing scenes. Moreover, there is a need in the art for a sensor which is self-adjusting to view scenes that are periodically illuminated.